It is well known in the art to prepare polyurethane elastomers by reacting excess polyisocyanate with a polyol to form a liquid or low melting polyurethane prepolymer having terminally reactive isocyanate groups, which can then be reacted with a diamine curing agent to form a polyurethane elastomer. Unfortunately, some of these elastomers exhibit poor "compression set", which makes them undesirable for use in molded articles which are to be used under load as in machine rolls and wheels.
For such load-bearing uses it is essential that the elastomer exhibit high dimensional stability so as to retain its resilience and not form flat spots under extended use. This property is known in the art as "good compression set". Compression set is commonly determined by ASTM Method D395 and represents the amount of residual deformation that occurs when an elastomer sample is placed under load sufficient to reduce its height to a specified value, held under that load for a certain period of time at a constant temperature, and then released from the load. Thus, if the height before sample loading is 100 units and it is compressed under load to a height of 90 units, but recovers to a height of 95 units when the load is removed, its compression set is 50%, i.e., 50% of the deformation was permanent. Obviously, the lower the compression set value for a given elastomer, the better it is for load bearing uses.
U.S. Pat. No. 3,997,514 discloses a process for preparing polyurethane elastomers having a compression set of 50% or less. In the process an isocyanate terminated prepolymer is prepared and vulcanized by the steps of:
1. Encapping an aromatic diisocyanate, such as 2,4-tolylene diisocyanate, with a glycol, such as poly(tetramethylene ether)glycol (M.W. about 1,000)
2. reacting the resulting intermediate with an aliphatic diisocyanate to form a prepolymer, and
3. curing the prepolymer with a suitable diamine to form a vulcanizate having improved compression set, specifically 50% or less.
Examples of aromatic diisocyanates are given in the penultimate paragraph of column 1, and examples of aromatic polyamines useful as cross-linking agents are given in the penultimate paragraph of column 2. Tolylene diamines are among the examples of polyamines enumerated in this paragraph, as well as 4,4'-methylenedianiline.
U.S. Pat. No. 3,428,610 relates to cross-linked polyurethane plastics and to a method of preparing the same. The cross-linked polyurethanes are prepared by reacting (1) one equivalent of an intermediate containing between 0.5% and 15% free NCO groups with (2) between 0.8 to 1.2 equivalents of a liquid aromatic diamine. The liquid aromatic diamine has (i) one linear alkyl substituent of 1 to 3 carbon atoms in an ortho position to one amino group and two linear alkyl substituents of one to three carbon atoms in both ortho positions to the other amino groups, or (ii) two linear alkyl substituents of one to three carbon atoms in both ortho positions of both amino groups. The patent discloses that any suitable organic polyisocyanate may be used, such as an aliphatic polyisocyanate having the formula EQU OCN--(CH.sub.2).sub.n --NCO
in which n=2 to 8 and aromatic diisocyanates. Examples of the polyisocyanates are given in the paragraph bridging columns 3 and 4. Among the examples of the suitable aromatic diamines that are set forth in the first full paragraph in column 4, are 1-methyl-3,5-diethyl-2,4-diaminobenzene and 1-methyl-3,5-diethyl-2,6-diaminobenzene. (Both of these isomers are also known as diethyltoluene-diamine or DETDA.) This reference, however, does not relate to the effect of the use of a particular aromatic diamine and its effect on the compression set properties of the resulting polyurethane elastomer.
U.S. Pat. No. 4,218,543 relates to a one-step process for the production of elastomeric moldings which have a compact surface by the technique of reaction injection molding, using highly reactive systems of specific polyisocyanates, polyhydroxyl compounds and specific active aromatic polyamines. This patent also relates to active hydrogen-containing blends useful in preparing elastomeric moldings, in which the blends comprise polyhydroxyl compounds, and specific active aromatic diamines. The elastomeric moldings are based on:
(a) 4,4'-diisocyanatodiphenylmethane and polyisocyanates based on 4,4'-diisocyanatodiphenylmethane;
(b) hydroxyl group containing compounds having a molecular weight of from 1,800 to 12,000 and preferably containing at least two primary hydroxyl groups;
(c) active aromatic amines as chain-lengthening agents;
(d) catalysts for the reaction between hydroxyl groups and isocyanate groups; and
(e) optionally, blowing agents, auxiliary agents and/or additives known in polyurethane chemistry. Among the examples of the aromatic amines are 1-methyl-3,5-diethyl-2,4-diaminobenzene and 1-methyl-3,5-diethyl-2,6-diaminobenzene.
This patent (U.S. Pat. No. 4,218,543) is directed towards reaction injection molding of polyurethanes where the gelation times are extremely fast (approximately 1 second or less). Based on the teachings of this patent, it would not be expected that diethyltoluenediamines (DETDA) could be utilized in polyurethane cast elastomers which require gel times of the order of several minutes. In particular, it would not be expected that good compression set properties could be obtainable with a particular polyurethane prepolymer when DETDA is used as the curing agent to produce the polyurethane elastomer.
Good compression set is achievable in the known cast elastomer art, wherein, for example, toluene diisocyanate is the diisocyanate used to prepare the prepolymer, and curing is effected with methylenbis(o-chloroaniline)[MBCA, also known as MOCA.RTM. brand of methylenebis(o-chloroaniline)]. The principal drawback of this system is the requirement for use of MBCA, which has been reported to be a cancer suspect agent based on tests with laboratory animals.
A welcome contribution to the art would be a polyurethane castable elastomer which has good compression set properties--i.e., a compression set value of less than or equal to 40%--and does not require the use of MBCA. Such a contribution is provided by this invention.